How can one tell a sister who hates the hijab that it is a departure from the religion?

You must help your sister wear the hijab by all possible means, encourage her gently, and explain the religious proofs for its obligation. Not wearing the hijab is a great sin, but it does not expel one from the fold of Islam. However, hating the teachings of Islam or hating the religion itself is blatant disbelief that nullifies deeds and expels one from the fold of Islam.